Spectrophotometers for measuring the composition of a substance by absorption spectroscopy are well known. For example, oximeters are used to determine concentrations of various hemoglobin components or fractions in blood samples from measuring an absorption spectrum in the visible and/or infrared wavelength range. Such an oximeter is disclosed in EP 210417.
In absorption spectroscopy, determination of a spectrum of a fluid sample is performed by transmission of light through a cuvette containing a part of the sample.
Absorption spectroscopy is based on Lambert-Beer's law according to which the absorbance determined for a sample containing a single optically active component (a dye) is directly proportional to the concentration of the component and the length of the light path through the sample in the cuvette:A(λ)=ε(λ)cd  (1)in which                A(λ) is the determined absorbance at wavelength λ,        ε(λ) is the molar extinction coefficient for the component at wavelength λ,        c is the molar concentration of the component, and        d is the length of the light path through the cuvette holding the sample.        
The absorbance A(λ) of the sample is defined as the logarithm of the ratio of the light intensity before and after transmission through the sample. In practice the absorbance A(λ) is defined as the logarithm of the ratio between the light intensity, I0, transmitted through a transparent aqueous reference solution and the light intensity transmitted through the sample:                               A          ⁡                      (            λ            )                          =                  log          ⁢                                    I              0                        I                                              (        2        )            
For samples containing more than one optically active component, the total absorbance Atotal is the sum of the individual components' absorbances since absorbance is an additive quantity. Thus, while Y optically active components in a sample the total absorbance is given by                                           A            total                    ⁡                      (            λ            )                          =                              ∑                          y              =              1                        y                    ⁢                                           ⁢                                                    ɛ                y                            ⁡                              (                λ                )                                      ⁢                          c              γ                        ⁢            d                                              (        3        )            
In a sample spectrum, the absorption Atotal(λ) recorded at each wavelength λ contains contributions from each of the components in the same. The magnitude of this contribution and thereby the concentration of each component in the sample is determined according to                               c          y                =                              ∑                          j              =              1                        J                    ⁢                                           ⁢                                                    K                y                            ⁡                              (                                  λ                  j                                )                                      ⁢                                          A                total                            ⁡                              (                                  λ                  j                                )                                                                        (        4        )            in which                J is the total number of wavelengths λj at which absorption is determined by the spectrophotometer and Ky(λj) is a constant specific for component y at wavelength λj.        
The vectors Ky(λ) may be determined mathematically by using methods such as multivariate analysis, or solving n equations with n unknowns, on data from reference samples.
It is also known to monitor performance of spectrophotometers, such as oximeters, by a measuring the absorption spectrum of a fluid quality control sample, QC sample, with the spectrophotometer in question.
Known quality control samples specific for blood analysis are typically red dye based samples designed to simulate the spectrum of blood. In addition to a red dye, they sometimes contain certain amounts of oxygen, carbon dioxide, and electrolytes at an established pH for determining performance of blood gas and electrolyte instruments. Synthetic QC samples having an absorption spectrum that closely mimics that of physiological blood have not yet been provided.
Quality control of the spectrophotometers, such as an oximeter, is typically performed by measuring the absorption spectrum of a QC sample comprising three to four different dyes. The dyes are mixed in a proportion so that the QC sample absorption spectrum mimics the absorption spectrum of blood. A spectrum of a QC sample is measured on the oximeter to be monitored and the parameter values determined by the oximeter are compared with predetermined control limits assigned to the QC sample by a qualified person. If the determined parameters are outside the corresponding control limits, servicing of the oximeter is required.
In WO 96/30742 a quality control method for monitoring performance of an oximeter is disclosed. The method comprises measuring the absorption spectrum of a QC sample and comparing it to a standard spectrum of the QC sample. Instrumental errors of the oximeter are considered to be the primary source contributing to the observed difference. Instrumental errors are converted into blood component concentration values to that instrument errors can be reported in terms understood by the operator of the instrument.
It is an important disadvantage of known quality control methods that, typically, known QC samples comprise 3-4 different dyes, causing long-term stability of the sample to be less than desired. To compensate for this, parameter value acceptance ranges in an oximeter may be widened leading to a more relaxed performance monitoring than desired.
It is another important disadvantage of known quality control methods that it is impossible with known quality control methods to distinguish between different types of instrument errors and to determine an individual contribution to deviation in parameter values from a specific type of instrument error. Thus, parameter value acceptance ranges have to be sufficiently wide to accommodate any possible type of instrument error. Further, a quality controlled spectrophotometer cannot be diagnosed if the determined parameter values lie outside the acceptable ranges. For example, a defect spectrophotometer with a wavelength shift may introduce the same deviation in the determined parameters as seen by dilution of the QC sample.
Future spectrophotometers are expected to facilitate determination of absorption spectra with improved resolution whereby instruments of higher precision and specificity are provided. High resolution measurements of spectra makes it more difficult to develop a suitable QC sample since precision and long term stability requirements are increased.
One of the most significant errors occurring in spectrophotometers is a wavelength shift. Due to manufacturing tolerances and drift during use, each spectrophotometer positions a determined spectrum slightly differently along the wavelength axis. Therefore, the wavelengths at which absorbance is determined are also positioned slightly differently for different spectrophotometers and thus, determined absorbances will vary for different spectrophotometers.